In order to achieve high efficiency, a high turbine inlet temperature is used in standard gas turbines. As a result, there can arise high NOx emission levels and higher life cycle costs. This can be mitigated with a sequential combustion cycle, wherein the compressor delivers nearly double the pressure ratio of a conventional one. The main flow passes the first combustion chamber (e.g. using a burner of the general type as disclosed in EP 1 257 809 or as in U.S. Pat. No. 4,932,861, also called EV combustor, where the EV stands for environmental), wherein a part of the fuel is combusted. After expanding at the high-pressure turbine stage, the remaining fuel is added and combusted (e.g. using a burner of the type as disclosed in U.S. Pat. Nos. 5,431,018 or 5,626,017 or in US 2002/0187448, also called SEV combustor, where the S stands for sequential). Both combustors contain premixing burners, as low NOx emissions involve high mixing quality of the fuel and the oxidizer.
Since the second combustor is fed by expanded exhaust gas of the first combustor, the operating conditions allow self ignition (spontaneous ignition) of the fuel air mixture without additional energy being supplied to the mixture. To prevent ignition of the fuel air mixture in the mixing region, the residence time therein should not exceed the auto ignition delay time. This criterion can ensure flame-free zones inside the burner. This criterion can pose challenges in obtaining appropriate distribution of the fuel across the burner exit area.
SEV-burners are currently designed for operation on natural gas and oil only. Therefore, the momentum flux of the fuel is adjusted relative to the momentum flux of the main flow so as to penetrate in to the vortices. This is done by using air from the last compressor stage (high-pressure carrier air). The high-pressure carrier air is bypassing the high-pressure turbine. The subsequent mixing of the fuel and the oxidizer at the exit of the mixing zone is just sufficient to allow low NOx emissions (mixing quality) and avoid flashback (residence time), which may be caused by auto ignition of the fuel air mixture in the mixing zone.